This invention relates to a valve lifter for an engine and more particularly to an improved valve lifter and method of manufacturing such a valve lifter.
A wide variety of types of valve lifters are employed in the valve actuating mechanism for internal combustion engines. Such lifters generally are cylindrical elements that are mounted for reciprocation within a bore in a component of the engine and are contacted at one end either by the cam lobe or a member actuated by the cam lobe and at their other end contact either the tip of the valve stem or another element that cooperates with the valve to open it. As is well known, it is very desirable to maintain a low weight in the reciprocating masses of the valve train so as to permit high speed operation and higher performance. Therefore, it is desirable to ensure that such valve lifters are formed from a lightweight material, such as aluminum, magnesium or titanium.
However, many of the lightweight materials and particularly those named do not have a good wear resistance. Either or both contact ends of the valve lifter are subject to wear and if these elements wear too rapidly then frequent lash adjustment is required to maintain the appropriate clearances in the valve system and to avoid noise.
One commonly used type of valve lifter employed with overhead camshaft engines consists of a thimble tappet which has a generally cylindrical configuration and is supported within a bore in the cylinder head assembly. The head of the tappet is engaged either by a cam or rocker arm actuated by a cam for reciprocating the tappet. The tappet has an internal surface that is engaged with the tip of the valve for transmitting this reciprocating motion to the valve. Normally, an adjusting shim is positioned between the head of the tappet and the cam. As with the general type of lifter problems mentioned above, this type of tappet also can be subject to wear in the area where the tappet engages the valve stem due to the high unit loadings.
It has been proposed to employ lightweight tappet bodies for this purpose and insert a form of hardened wear-resistant element between the tappet body and the valve stem. However, if this hardened element is not fixed rigidly relative to the tappet body, then relative motion can occur which will cause wear of the tappet body, noise and variations in clearance.
It has been proposed to employ a non-circular hardened element that is received in a non-circular recess formed in the tappet body so as to hold the hardened element and tappet body together and against rotation. However, such constructions have a number of disadvantages.
Specifically, it is difficult to form non-circular recesses in the tappet body. In addition, to form the non-circular hardened insert, a stamping process is frequently employed. However, as is well known with stamping or punching operations, the thin material that is being punched will tend to be deformed at the peripheral edges so that a flattened surface does not result. Thus, either the insert must be subsequently machined to bring the surface flat or the insert will tend to flatten out in operation and the clearances change with the aforenoted problems.
Furthermore, it is difficult to machine the recess in the tappet body to have a sharp edge at the base of the opening that receives the insert. Hence, this type of construction is not particularly advantageous.
It is, therefore, a principal object of this invention to provide an improved valve lifter that can be made from a composite material in a low cost method and which will have the hardened insert be rigidly held in place throughout the whole life of the tappet to avoid wear.
It is a further object of this invention to provide an improved valve lifter made of a composite material wherein the hardened insert can be easily interlocked into the tappet body and wherein circular surfaces may be employed to avoid the aforenoted defects.
It is a still further object of this invention to provide an improved tappet construction for an engine and method of manufacturing it which permits light weight and long life.